Faraday rotation measurements by phase-based technique on HL-2A

Abstract

Poloidal magnetic field is a very important physical parameter for the understanding of heating and confinement in tokamak plasmas. One channel of an eight-chord, horizontally-viewing, double-path interferometer system on HL-2A has been modified to include a polarimeter capability in order to measure Faraday rotation. The polarimeter utilizes one phase technique, which is based on a Veron-type HCN laser interferometer and ``Dodel and Kunz''-type polarimeter including a rotating grating to shift the frequency of one probing beam by Δω, and two counter-rotating circularly-polarized probing beams. The Faraday rotation angle can be directly determined by measuring the plasma birefringence. The implementation of this instrument only needs one HCN laser source and one detector to characterize the rotation. The first experimental results have shown that the Faraday rotation angle of less than 1° can be measured with up to 0.1 ms time resolution.